U.S. Pat. No. 3,243,211 ("Wetmore") discloses a connector containing a fusible material so that upon insertion of an object to be joined to the connector or upon insertion into the connector of two members to be joined, and upon heating of the connector, the fusible material is caused to melt and to contact the object or objects and to effect a bond upon cooling. The connector may also include a heat-recoverable member whereby the liquified fusible material is bounded and caused to contact the object or objects while in the fluid state. This device requires an external heat source such as hot air or an infrared radiant source in order to melt the fusible material.
A problem with the Wetmore device is that it can cause overheating of objects to be soldered or otherwise bonded as well as adjacent objects. In the electronics art, for instance, overheating of delicate integrated circuits is a problem as is overheating of circuit boards, mastics, resins, heat-shrinkable polymers, glues, potting compounds--all of which can be degraded or destroyed by the application of excessive heat. Further, the Wetmore device has little utility for joining wires, tubes or members which are large effective heat sinks since the large amount of heat required cannot be readily transferred through the heat-shrinkable sleeve without damaging it.
U.S. Pat. No. 4,914,267 ("Derbyshire") relates to connectors containing fusible materials to assist in forming a connection, the connectors forming part of a circuit during the heating of the fusible material In particular, the temperature of the connectors is autoregulated at about the Curie temperature of the magnetic material included in the circuit during the heating operations. The connector may be a ferromagnetic member or may be a part of a circuit including a separate ferromagnetic member.
Derbyshire explains that autoregulation occurs as a result of the change in value of mu (a measure of the ferromagnetic properties of the ferromagnetic member) to approximately 1 when the Curie temperature is approached. In particular, the current spreads into the body of the connector thus lowering the concentration of current in a thin layer of magnetic material, and the skin depth changes by at least the change in the square root of mu. Resistance to current flow reduces, and if the current is held at a constant value, the heating effect is reduced to below the Curie temperature, and the cycle repeats. Thus, the system autoregulates about the Curie temperature.
Derbyshire discloses embodiments wherein the connector is made of ferromagnetic material and a high frequency constant a.c. current is passed through the ferromagnetic material causing the connector to heat until its Curie temperature is reached. When this happens, the effective resistance of the connector reduces and the power dissipation falls. By proper selection of current, frequency and impedance, and proper selection of thickness of materials, the temperature is maintained at about the Curie temperature of the magnetic material of the connector.
In another embodiment of Derbyshire, the connector is made of a highly conductive, nonmagnetic material, and a crimping tool having ferromagnetic jaws is used to heat the connector by supplying a high frequency, constant current to opposite ends of the jaws. In a further embodiment, a laminar ferromagnetic-non-magnetic heater construction comprises a copper wire, tube, rod or other metallic element in a ferromagnetic sleeve. In this case, current at proper frequency applied to opposite ends of the sleeves flows through the sleeve due to the skin effect until the Curie temperature is reached, at which time the current flows primarily through the copper wire. In a still further embodiment of Derbyshire, the connector includes a copper sleeve with axially-spaced rings of high mu materials of different Curie temperatures so as to produce different temperatures displaced in time and space.